Compressionally-loaded spring forming dc connection between the outer and inner conductor of a tem-mode transmission line



Jan. 21, 1969 T. J. RUSSELL 3,423,702

COMPRESSIO LY-LOADED SPRING FORMING DC CONNECTION BETW THE OUTER AND INN CONDUCTOR OF A TEM-MODE TRANSMISS N LINE Filed Dec. 1, 1966 INVENTO THOMAS J. RUSS BY W LLQM ATTORNEY United States Patent 8 Claims ABSTRACT OF THE DISCLOSURE 'Oftentimes it is necessary to provide means for making a -DC connection between the inner and outer conductors of a high-frequency transmission line for transrnitting a TEM-mode wave to bias or otherwise control active elements, such as PIN diodes, shunted across or serially connected into the line. Such DC connectors ideally should not disturb the :RF characteristic of the transmission line, and should provide a good direct current path. The DC connector described herein is in the form of a conductive, helical coil compression spring which is axially compressed between the two conductors between which the TEM-mode is propagated.

Background of the invention Field of the inventi0n.This invention relates to direct current (DC) conductive connections between the conductors of TEM-mode transmission lines and, more particularly, to such a connection :which has substantially no effect on the radio frequency (RF) transmission characteristics of the transmission line.

Description of the prior art.-Heretofore a number of means have been utilized to make a DC connection between the conductors of a TEM-mode transmission line such as, for example, the inner and outer conductor of a coaxial line, but none of them has been found entirely satisfactory in that such means were either narrow band, or difiicult to install, or both. For example, one of the means utilized heretofore as a DC connection !was the employment of a quarter-wave shorted stub which provides good DC conduction, but the resultant RF isolation is good only at the RF signal corresponding to the one for 'which the stub is truly a quarter-wave long. At other frequencies, the RF isolation decreases making the transmission line with a quarter-wave stub a narrow band device.

Another technique that has been employed heretofore is the utilization of a piece of fine wire formed into a helical or spiral coil and having its ends connected to the inner and outer conductors of the transmission line, respectively. Both these coil connectors provides excellent DC conduction, and are much more broadband in RF isolation than the quarter-wave stub. Particularly, the helical coil connector has been found to provide less than a three-quarter db loss over a frequency range of 10 octaves, and to introduce a VSWR of less than 1.5. The problem encountered with such fine wire coil connectors is the difliculty and, therefore, the expense of installation "ice since the coil ends must be soldered to the inner and outer conductors for good DC contact. Another disadvantage of the fine coil connectors is that, after installation, the inner and outer conductors of the transmission line can neither be translated nor rotated with respect to one another since any motion therebetween will cause the fine wire to break.

Summary of the invention The present invention overcomes these problems by utilizing a fine wire coil compression spring which is axially supported in a radial bore drilled through the dielectric spacer separating the two conductors of the transmission line and end supported by compression between the two conductors respectively. In this manner, good electrical contact between the compression spring connector and the two conductors is established by the spring force. In case of symmetrical coaxial transmission lines, the inner and outer conductor may be rotated and translated with respect to one another without breaking the electrical connection. Further, installation is extremely simple since the coil spring only needs to be compressed initially into the radial bore provided in the dielectric spacer and slipped into place in the transmission line.

Brief description of the drawings FIGURE 1 is a cross section view of a portion of a coaxial transmission line and its N-type connector having a DC connection in accordance with the present invention;

FIGURE 2 is a section taken along lines 2-2 of FIG- U-RE 1; and

FIGURE 3 is an alternative embodiment of the invention in which the DC connection in the coaxial transmission line is isolated from the outer conductor and is between the inner conductor and a connection post telescoping from the outer conductor.

Description of the preferred embodiment Referring now to the drawings, there is shown the end portion of a coaxial transmission line 10 terminated in a type-N connector 11. As far as pertinent hereto, transmission line portion 10 comprises an outer conductor 12, an inner conductor 14 and a dielectric spacer 16 supporting inner conductor 14 within outer conductor 12. Spacer 16 includes a radial bore 1 8 in which is disposed the direct current connection, between the outer conductor and the inner conductor, in the form of coil compression spring 20.

Coil compression spring 20 is constructed out of a conductive spring material, such as stainless steel wire, and has a free length (uncompressed state) which is greater than the radial gap width between outer conductor 12 and inner conductor 14. Spring 20 is axially supported by bore 18 and its ends press against the exposed portions, on either end of bore 18, of the inner and outer conductors respectively, thereby making a good conductive spring contact with the conductors.

In selecting the physical configuration of the spring, it is desirable to have as many coils as possible and a coil diameter as large as possible for maximum inductive impedance to the RF energy transmitted through the transmission line. To obtain the largest number of coils and, therefore, the largest inductance, the fully compressed length of spring 20 is made slightly smaller than the radial gap distance between outer conductor 12 and inner conductor 14 so that, when the same is placed in position, as shown in FIGURE 1, adjacent coils do not touch. Also, the diameter of the coil wire itself, which also determines the maximum number of coils of the spring, is held to a minimum commensurate with its required current carrying capacity and providing enough spring force to make good contact at the spring ends. The outside coil diameter is selected as large as possible, but it has bene found that the diameter need not be larger than the diameter of inner conductor .14 for good RF isolation and, in fact, may be considerably smaller.

Spring 20 is easily installed into transmission line by removing dielectric spacer 16, which forms a standard part of a type-N connector, and drilling bore 18 therethrough. Thereafter, spacer 16 is slipped over conductor 14 a short distance, and spring 20 is inserted into bore 18 and compressed. Thereafter, spacer 16 with the compressed spring in place is slipped into its original position, and the DC connector of this invention is installed. Rotational or translational motion will not tear or affect the DC connector.

While the preferred configuration of spring 20 is helical, it is to be understood that configurations such as convolute or conical, either single or double, may likewise be utilized. However, the helical configuration provides the maximum impedance for a given line gap length and outside diameter spring resulting in maximum isolation which is the primary reason why this configuration is preferred.

By way of example, for a 50-ohm rexolite dielectric transmission line Whose inner conductor has a diameter equal to .0885 inch and an outer conductor having an inner diameter of .3365 inch and, therefore, a radial gap equal to .1240 inch a DC connector has been constructed by utilizing a spring commercially available from Lee Spring C0,, Brooklyn, N.Y., Item No. CI-006A6 which comprises a 6-mil diameter stainless steel wire having an outside diameter of .057 inch, a free length of 7 inch, and a minimum compressed length of .108 inch. The diameter of bore 18 in dielectric 16 was .062 inche.

Tests conducted with this particular DC connection showed that the RF transmission characteristics of the coaxial transmission line were substantially unaffected over a frequency band ranging from 1,000 mHz. to 12,000 mHz., and the changes in the VSWR and the insertion loss were substantially negligible.

Referring now to FIGURE 3, there is shown an alternate embodiment of the DC connector of this invention in which a DC connection is made to the inner conductor of a coaxial line through and isolated from the outer conductor. The transmission line thereshown comprises an inner conductor 30 which is supported within an outer conductor 32 by a dielectric spacer 34 which includes a bore 36 for accommodating a coil compression spring 38. Spring 38 is similar to the one described in connection with FIGURES 1 and 2. Outer conductor 32 is provided with a bore 40 which is usually, though not necessarily, coaxial with bore 36, and is dimensioned to accommodate an aluminum plunger 42 having a connector post 34 to which a DC connection can be made. Plunger 42 is held in place by an insulator 44 which is fastened to outer conductor 32 by means of fastening screws 46. The cylindrical surface of plunger 42 in contact with bore 40 is anodized to provide DC insulation to the outer conductor and a RF short to the wave energy propagated by the transmission line.

Installation of DC connector 38 is readily accomplished by removing plunger 42, inserting spring 38 and replacing plunger 42 by forcing the same into bore 40 against the compression exerted by spring 38. Plunger 42 is secured in place by clamping the retainer plate 44 to outer conductor 32 by means of screws 46.

While the drawings show a cylindrical coaxial transmission line, it is to be understood that the DC connection of this invention is equally applicable to other types of coaxial transmission lines such as unsymmetrical transmission lines in which the inner conductor or the outer conductor, or both, have cross section shapes other than circular or in which the inner conductor is not coaxial with the outer conductor. The term coaxial transmission line, as used herein, is intended to include all transmission lines in which an outer conductor surrounds an inner conductor Whatever their shape or spacing. Further, the present invention is also useful for strip lines in which a pair of flat conductors are placed in spaced relation with one another. In such strip lines, the DC connector of this invention is compressed between the two spaced conductors. The term TEM-mode transmission line is meant to include such coaxial lines and such strip lines.

There has been described a DC connector for use in a coaxial transmission line which is mechanically rugged, economical to install and provides excellent DC conduction and RF isolation.

What is claimed is:

1. A direct current conductive connection between the conductors of a TEM transmission line which has a negligible effect on the transmission lines RF properties, said conductive connection comprising, in combination:

an axially elongated, conductive, coil compression spring axially compressed between the transmission line conductors to establish good conductive pressure contacts between opposite ends of said spring and the transmission line conductors, respectively; and

a dielectric spacer disposed between the transmission line conductors having a bore therethrough which supportively accommodates said spring.

2. A conductive connection in accordance with claim 1 in which said coil spring is a helical spring of cylindrical configuration.

3. A conductive connection in accordance with claim 1 in which said coil spring is shaped for providing maximum inductive impedance to the RF energy propagated along the transmission line.

4. A conductive connection in accordance with claim 1 in which the number of coils in said coil spring is selected to be as large as possible without causing conductive contact between adjacent coils when compressed in position between the transmission line conductors.

5. A direct current conductive connection between one conductor of a TEM transmission line which passes through and is isolated from the other conductor of the transmission line and which has a negligible effect on the transmission line RF properties, said conductive connection comprising:

an opening in said other conductor;

terminal post means carried by said other conductor and passing through said opening, said post means including isolation means for direct current isolation from said second conductor and a surface facing said one conductor; and

an axially elongated, conductive, coil compression spring axially compressed between said one conductor and the surface of said post means whereby good conductive pressure contacts between opposite ends of said spring and said one conductor and said post means, respectively, are established.

6. A direct current conductive connection in accordance with claim 5 in which said surface is shaped and disposed to generally conform to and be flush with the interior surf-ace of said other conductor and in which the separation between said other conductor and said post means is sufiiciently small to form an RF short to the wave energy propagated by the transmission line.

7. A direct current conductive connection in accordance with claim 5 which further includes a dielectric spacer between the conductors forming the transmission line, and in which said dielectric spacer includes a bore therethrough dimensioned for accommodating said coil spring, opposite ends of said bore being disposed opposite and immediately adjacent said one conductor and said surface, said coil spring being axially disposed in said bore.

8. A direct current conductive connection in accordance with claim 7 in which said one conductor is the center conductor of a coaxial transmission line and said other conductor is the outer conductor of the coaxial transmission line, and in which said bore is radial to the axis of said coaxial transmission line.

6 References Cited UNITED STATES PATENTS 2,901,613 8/1959 Patterson et al. 329-162 X 2,922,955 1/1960 Leboutet 329-461 X OTHER REFERENCES Garver, R. V.: Theory of TEM Diode Switching, IRE Trans. on MTT, May 1961, pp. 232-234 relied on.

10 HERMAN KARL SAALBACH, Primary Examiner.

PAUL L. GENSLER, Assistant Examiner.

US. Cl. X.R. 33620 

